The field of the invention is nuclear magnetic resonance imaging ("MRI") methods and systems. More particularly, the invention relates to the removal of artifacts in MR images produced by changes in the polarizing magnetic field during the acquisition of data.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at a characteristic Larmor frequency which is determined by the gyromagnetic constant .gamma. of the spins and the polarizing magnetic field B.sub.0. If the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. A signal is emitted by the excited spins, and after the excitation signal B.sub.1 is terminated, this NMR signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G.sub.x G.sub.y and G.sub.z) are employed. Typically, the region to be imaged is scanned by a sequence of separate measurement cycles (referred to as "views") in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well-known reconstruction techniques.
A well-known problem with MRI systems is variations in the strength of the polarizing magnetic field B.sub.0. Such variations affect acquired images in two ways. First, changes in B.sub.0 cause corresponding changes in the phase of the acquired NMR signals. Such spurious phase changes appear in the acquired NMR signals, or "k-space data", and result in ghosting or blurring artifacts in an image reconstructed using the Fourier transformation method. Since the spurious phase shift accumulates continuously between RF excitation and data acquisition, the artifacts are particularly troublesome with gradient recalled echo pulse sequences having a long echo time, TE. Changes in B.sub.0 can also cause apparent spatial shifts along the frequency encoding (i.e. readout) gradient direction.
The second deleterious effect of changes in polarizing magnetic field B.sub.0. occurs when slice selection techniques are used in the pulse sequence. The change in B.sub.0 shifts the location of the excited slice by an amount equal to the change in Larmor frequency divided by the bandwidth of the selective RF excitation pulse. For example, if B.sub.0 shifts the Larmor frequency by 20 Hz and the selective RF excitation pulse has a bandwidth of 1000 Hz, the excited slice will shift 2% from its expected position along the slice select gradient axis. Such shifts can cause amplitude changes in the acquired data.
Many methods are used to control and regulate the polarizing magnetic field B.sub.0. Most of these methods deal with changing conditions within the scanner itself and are quite effective. For example, methods for compensating the effects on B.sub.0 due to Eddy currents produced by changing magnetic field gradients are disclosed in U.S. Pat. Nos. 4,698,591; 5,289,127; and 5,770,943.
Polarizing magnetic field strength B.sub.0 is affected by external events such as the movement of large masses of metal in the vicinity of the scanner. Moving objects such as cars, trucks, trains and elevators can change the polarizing magnetic field and produce image artifacts.
Two methods have been used to reduce the effects of such disturbances, passive methods and active methods. Passive methods include the use of shielding materials around the main magnet as described, for example, in U.S. Pat. No. 4,646,046. Massive amounts of silicon steel sheets are placed around the magnet, which results in an expensive, heavy and difficult to install system.
Active compensation systems employ a sensor which measures the change in magnetic flux at a location near the scanner and uses this information to compensate the system. Such compensation may include producing a current in a coil that generates an offsetting correction magnetic field. Such methods employ flux sensors as described, for example, in U.S. Pat. No. 5,952,734 or ESR instruments as disclosed in U.S. Pat. No. 5,488,950. These active methods do not work well when the field disturbance is produced by multiple sources or sources of varying magnitude or location.